FIG. 13 is a view showing a conventional 1/(N/2) frequency divider, that is, a 1/2.5 divider when N is 5. In FIG. 13, reference numerals 10, 20 and SO each designate a master-slave type flip-flop (referred to as an M-S F/F hereinafter) and reference numerals 41 and 42 each designate an OR circuit. Reference T designates a dividing signal input terminal and reference OUT designates a dividing signal output terminal. In addition, references D, Q, Q, and T shown in M-S F/F designate a data input terminal, an output signal terminal, an inverted signal output terminal and a clock signal input terminal of the M-S F/F, respectively. Reference numerals N11 to N42 each designate the potential of a corresponding signal line, in which the N11 and the N12 are an output signal and an inverted output signal of the M-S F/F 10, respectively, the N22 is an inverted output signal of the M-S F/F 20, the N31 and the N32 are an output signal and an inverted output signal of the M-S F/F 30, and the N41 and the N42 are output signals of the OR circuits 41 and 42, respectively.
Next, operation thereof will be described hereinafter.
Each M-S F/F constitutes a delay type flip-flop, in which a signal input to the data signal input terminal D is output in synchronization with a clock signal. In the M-S F/F's 10 and 20, when the signal N31 input to the OR circuit 42 is "High", 1/2 dividing operation is performed and the input signal to the dividing signal input terminal T is divided into 1/2, so that a 1/2 dividing output can be obtained from the N11 . When the signal N31 is "Low", the M-S F/F's 10 and 20 perform 1/3 dividing operation and its output signal OUT is obtained from the N11. In addition, the M-S F/F 30 performs 1/2 dividing operation and its output is obtained at the N31.
Next, operation of this frequency divider will be described hereinafter in reference to FIG. 14. FIG. 14 shows signal waveforms of the signal lines of the frequency divider. In FIG. 14, each reference designates a waveform of a signal of a terminal or a potential shown in FIG. 13. One "High" interval of the N31, signal corresponds to one period of the N11 and to two periods of the T. Since the signal of the N31 is inverted for one period of the N11, the N31 becomes "Low". One "Low" interval of the N31 corresponds to three periods of the T. Then, the N31 becomes "High" after three periods of the T signal. Thus, the above operation is repeated. Since the output of the frequency divider is the N11, the frequency divider repeats 1/2 dividing operation and 1/3 dividing operation and then the output signal OUT having one period in five periods of the dividing input signal T can be obtained. Thus, 2/5, that is, 1/2.5 frequency divider can be obtained.
Next, a 1/3 frequency divider when the N is 6 will be described hereinafter as a conventional example of the 1/(N/2) frequency divider.
In FIG. 15, the same references as in FIG. 13 designate the same elements or elements having the same functions. The 1/3 frequency divider is different from the 1/2.5 divider shown in FIG. 13 in that one M-S F/F and one OR circuit are omitted.
Next, operation thereof will be described hereinafter.
The 1/3 frequency divider performs the same operation as that when the N31 of the frequency divider shown in FIG. 15 is always "Low". In addition, operation of the frequency divider will be described in reference to FIG. 16. FIG. 16 shows signal waveforms on corresponding signal lines of the frequency divider. In FIG. 16, each reference designates a waveform of a signal at a terminal or a potential shown in FIG. 15. One period of the output signal OUT (N11) corresponds to three periods of the T. The OUT signal is "High" for two periods of the T. Thus, a frequency divider capable of obtaining 1/(6/2), that is, 1/3 dividing output can be provided.
FIG. 17 is a view showing a conventional pulse signal former for obtaining a pulse signal whose duty ratio, that is, ratio of a pulse width to a period of pulse, is 1/2.
In FIG. 17, reference numeral 200 designates a bandpass filter and reference numeral 201 designates a pulse waveformer. Reference numeral IN1 designates an input signal, reference numeral N200 designates an output signal of a band-pass filter and reference OUT designates an output signal of the circuit.
Next, operation of this pulse signal former will be described hereinafter.
Since a sine wave whose period is a period of a pulse signal is only taken out from the input pulse signal IN1 by the band-pass filter 200, the difference in duty ratio between the pulse signals is disregarded. Then, since the sine wave is converted to a pulse signal by the pulse waveformer 201, the output signal OUT becomes a pulse signal whose duty ratio is 1/2.
Since the conventional frequency divider is formed as described above, when the N is an odd number (for example, N=5) in the 1/(N/2) frequency divider, only a signal whose period varies with time is obtained as the output signal OUT as shown in FIG. 14. In addition, the duty ratio of the output signal is not 1/2, which is a problem.
In addition, when the N is not a multiple of 4 (for example, N=6), the duty ratio of the output signal is also not 1/2.
Those facts cause distortion of a signal when an operation such as modulation, demodulation or orthogonal modulation is performed.
In addition, according to the conventional pulse signal former, since the band-pass filter 200 is used as shown in FIG. 17, the usable frequency is limited to the range of the operation frequency of the band-pass filter.